Impulse and Momentum

A 1550kg car, traveling with a velocity of 12m/s plows into a 1220kg stationary car. During the collision the two cars lock bumpers and then move together as a unit. (a) what is tehir common velocity just after the impact?
(b) What fraction of the initial kinetic energy remains after the collision.

(a) Pf=Pi

(m1+m2)vf=m1vi1
(1550kg+1220kg)vf=1550kg(12m/s)
2770kgvf=18600kg m/s

vf=6.71 m/s
I know my answer for part A is correct but part B i really have no idea how to approach it.

A 1550kg car, traveling with a velocity of 12m/s plows into a 1220kg stationary car. During the collision the two cars lock bumpers and then move together as a unit. (a) what is tehir common velocity just after the impact?
(b) What fraction of the initial kinetic energy remains after the collision.

(a) Pf=Pi

(m1+m2)vf=m1vi1
(1550kg+1220kg)vf=1550kg(12m/s)
2770kgvf=18600kg m/s

vf=6.71 m/s
I know my answer for part A is correct but part B i really have no idea how to approach it.

What is the initial kinetic energy of the system just before impact? What is the kinetic energy of the system immediately after impact? What is the ratio of the 2 values?